um: kernel: ksyms: Export symbol syscall() for fixing modpost issue

syscall() is implemented in libc.so/a (e.g. for glibc, in "syscall.o"),
so for normal ".o" files, it is undefined, neither can be found within
kernel wide, so will break modpost.

Since ".o" files is OK, can simply export 'syscall' symbol, let modpost
know about that, then can fix this issue.

The related error (with allmodconfig under um):

    MODPOST 1205 modules
  ERROR: "syscall" [fs/hostfs/hostfs.ko] undefined!

Signed-off-by: Chen Gang <gang.chen.5i5j@gmail.com>
Signed-off-by: Richard Weinberger <richard@nod.at>